Charged particle beam columns are typically employed in scanning electron microscopy (SEM), which is a known technique widely used in the manufacture of semiconductor devices, being utilized in CD metrology tools, the so-called CD-SEM (Critical Dimension Scanning Electron Microscope) and defect review SEM (DR-SEM). In an SEM, the region of an object to be examined is two-dimensionally scanned by means of a focused primary beam of electrically charged particles, usually electrons. Irradiation of the object with the primary electron beam releases secondary (and/or backscattered) electrons. The secondary electrons are released at that side of the object at which the primary electron beam is incident, and move back to be captured by a detector, which generates an output electrical signal proportional to the so-detected electric current. The energy and/or the energy distribution of the secondary electrons is indicative of the nature and composition of the object.
The inspection of an object with a DR-SEM requires a certain, relatively high tilt (as compared to that required for CD-measurements) of the object's surface with respect to the incident electron beam (e.g., 45 degree angle of incidence). When inspecting patterned objects, such as semiconductor wafers, having a pattern in the form of a plurality of spaced-apart grooves, tilting of the object is needed to detect the existence of a foreign particle located inside a narrow groove.
It is known to implement a tilt mechanism by either mechanically tilting the object's carrier with respect to the charged particle beam column or tilting the column with respect to the object's carrier, or both (e.g., U.S. Pat. Nos. 5,329,125; 5,734,164; 5,894,124; 6,037,589). It is also known to achieve a tilt by affecting the trajectory of the primary electron beam using single- or double-deflection, the so-called “electronic tilt” (e.g., WO 01/45136 and U.S. Pat. No. 6,380,54, assigned to the assignee of the present application).
One of the common goals of all imaging systems consists of increasing the image resolution. In SEM, in order to reduce the “spot” size of the electron beam up to nanometers, a highly accelerated electron beam is typically produced using accelerating voltages of several tens of kilovolts and more. Specifically, the electron optical elements are more effective (i.e. produce smaller aberrations) when the electrons are accelerated to high kinetic energy. However, it has been observed that such a highly energized electron beam causes damage to resist structures and integrated circuits, and, in the case of dielectric specimens, causes undesirable charging of the specimen. Therefore, the primary electron beam is decelerated just prior to impinging onto the object by an electric field created in the vicinity of the object. This electric field, while decelerating the primary electrons, accelerates secondary electrons released at the object.
The above can be implemented by using an lens arrangement in the form of a combination of a magnetic lens arrangement and an electrostatic lens (e.g., WO 01/45136, EP 1045425, U.S. Pat. No. 6,380,546, all assigned to the assignee of the present application, and WO 01/5056). The electrostatic part of such a compound magnetic-electrostatic lens is an electrostatic retarding lens (with respect to the primary charged particle beam), which has two electrodes held at different potentials, one of the two electrodes being formed by a cylindrical anode tube that defines a beam drift space and arranged within a magnetic lens arrangement along its optical axis, and the other electrode being a metallic cup provided below the magnetic lens arrangement. Generally, the electrostatic part of the lens arrangement may not be implemented as a separate electrostatic lens, but rather by applying appropriate voltages to the anode tube and the object, or to the anode tube, the polepiece of the magnetic lens arrangement and the object.
The operation with low primary beam energies (less than 1 keV), especially in large mechanical tilts (e.g., 45 degrees), limits the system resolution due to high chromatic aberration of the electrostatic part of the lens arrangement, namely, the distribution of the electrostatic field in the vicinity of the object. Due to the variance in energy, the primary beam particles are typically dispersed into beam components formed by, respectively, particles of average energy, particles of relatively high-energy, and particles of relatively low-energy. The high-energy particles are less diffracted than the low-energy particles. This difference causes the enlargement of the diameter of the charged particle beam, and consequently, the reduction of resolution. This is also referred to as chromatic aberrations of focusing. The low-energy beam is characterized by more expressed chromatic aberrations than the high-energy beam.
U.S. Pat. No. 2,001,0011702 discloses a technique of observing semiconductor wafers with higher resolution at a low acceleration voltage, in particular, when a wafer is inclined or tilted at large angles. This technique utilizes a composite lens consisting essentially of a single-pole or monopole magnetic field type lens and an electrostatic field invasive lens, whereas an electrode of the electrostatic field invasive lens which opposes the wafer is made of a magnetic material while letting a high voltage of the negative polarity be applied to this electrode and the wafer.